This invention relates to separating particles of different mass in a gas mixture. It has principal utility in the separation and size analysis of submicron particles in aerosols and in the isolation and enrichment of gaseous isotopes. It is to be understood that the term "particle" as used herein encompasses the entire size range from solid or liquid aggregates of about a micron in diameter to molecules of a few Angstroms in diameter.
Conventional methods of gas separation through gaseous diffusion, absorption, liquefaction, and fractional distillation are well known. Recent developments in gas separation technology, particularly as applied to isotope separation for uranium enrichment, have produced various forms of gas centrifuges and gasdynamic separation devices. As summarized in the article entitled "Isotope Enrichment by Aerodynamic Means: A Review and Some Theoretical Considerations" by R. Eaton, R. Fox, and K. Touryan, published in Journal of Energy, Vol. 1, No. 4, July-August 1977, pages 229 to 236, gasdynamic separation devices are generally characterized either as Type I, wherein the separation of species of different mass is induced by pressure gradient, or Type II, wherein separation is achieved through molecular perturbations from an equilibrium distribution. For separation of gaseous isotopes, conventional methods and present gasdynamic separation schemes are generally characterized by the problems of low separation yields, high capital costs, high power consumption per unit of separation work, technical complexity, and/or lack of compatibility with available equipment.
In a related field, the economical separation of different constituent particles in aerosol mixtures according to mass has also not been fully realized. The resolution of many environmental and health problems depends upon the separation and quantitative size analysis of aerosols. An apparatus capable of reliably discriminating between various size particle species and measuring their absolute concentrations would find immediate use in such diverse areas as environmental pollution control, research on respiratory illness, and monitoring and control of industrial processes. The need for such apparatus becomes even more acute, for example, as coal assumes a more important role as an energy source.
There is also a need for an effective means of mass analysis for industrially manufactured and naturally occurring polymers, crystallites, microemulsions, and the like, which have dimensions in the submicron range. An important application for a particle separation method or apparatus would therefore include the mass analysis of such material whenever it is possible to disperse them in an aerosol.
Gasdynamic methods for separating two species of disparate mass employ acceleration or deceleration of a flow to produce a differential drift velocity between species. It is this drift velocity which results in spatial separation. The potential for separation at any point is measured by the separation speed ratio, defined as the ratio of drift velocity between the two species of interest divided by their random thermal velocity. Operating conditions which increase this speed ratio increase the separation effect, but they also tend to increase the energy requirements of the device.
Full utilization of the separation potential inherent in a given separation speed ratio requires that the region where spatial separation of the particle species occurs be collision free. Most gasdynamic separation methods are Type I systems in which spatial separation or segregation occurs in a collision-dominated region. The separation factors obtained in such devices are quite low. A few Type II systems have been proposed which exhibit collisionless or free molecular flow in the separation region and as a result are characterized by larger separation factors. All present Type II systems, however, require the maintenance of very low background gas pressures to insure free molecular operation. Such devices have high energy requirements.
It is therefore a principal object of this invention to provide a simple means of achieving high levels of separation of particle species of disparate mass for aerosol analysis, isotope separation and enrichment, and other uses. It is also an important object that the inventive system require relatively low energy consumption per unit of separation work and be capable of simple, inexpensive, and convenient implementation. A further object of the invention is that the separation region be characterized by collisionless or free molecular flow at relatively high operating pressures.